Twenty-one hour light-dark cycle accelerates vaginal opening in the rat.

T he estrus cycle of rats and hamsters is regulated by the daily light-dar k cycle . The suprachiasmatic nucleus of the hypothalamus appears t o generate a rhythm with a frequency of approximately one cycle in 24 hours . This rhythm is exactly synchronized with the external light-dark cycle b y impulses received from the eyes through a retinohypothalamic projection . Lesions that destroy the suprachiasmatic nucleus at two days of age produc e constant vaginal estrus in rats and alter the time of vaginal opening i n blinded neonatal rats . 1 . 2 This paper reports a pilot study demonstrating that a 21-hour light-dar k cycle accelerates vaginal opening in rats, a finding which suggests that th e early menarche in blind girls3 and the accelerated vaginal opening of rat s reared in darkness 4 are most probably caused by acceleration of norma l 24-hour circadian rhythms .